The present invention relates to a data communications system and method utilizing packet framing techniques.
Serial data streams that use packet protocols to transfer information need to establish byte alignment of the data and mark the beginning and end of the packets.
In the past various methods have been used to accomplish these tasks. HDLC (High-level Data Line Control) protocols use a unique eight bit flag to mark a packet's start and end. Inside the packet the data is bit stuffed to prevent an accidental flag from being sent. The 802.3 "Ethernet" standard is typical of another set of protocols which use an idle preamble followed by a non-idle packet start header to mark the beginning of a packet.
These methods have worked well in applications where bandwidth efficiency has not been an issue. However in telecommunications, the customer typically rents bandwidth from a service provider. Protocols for this application have to maximize the bandwidth available for the customer's data to save money on the purchased service. The framing algorithm should have as low an overhead as possible. For example if 24 bytes of user data were carried in an HDLC framed packet, it could have as much as 27 bits of data stuffed into it to prevent a false flag.
To this would be added a start and end flag of eight bits each. Such a worst case packet would have a 22% packet framing overhead. For a Tl line which can cost hundreds or thousands of dollars a month, this is expensive.
StrataCom, Inc. manufactures a digital multiplexer (known as the Integrated Packet Exchange (IPX)) that connects customer's voice and data equipment together over Tl lines using a packet transport protocol. That IPX multiplier is described in U.S. Pat. No. 4,771,425, the details of which are hereby incorporated by reference.
Presently the IPX solves the problem of framing packets on Tl lines by making all of its packets 24 bytes long and placing them inside a Tl frame. The Tl frame format has one framing bit and 24 bytes of data; 193 bits total. The Tl framing bit is required for the line to use Tl vendor services. Therefore, this packet framing, which is the same as the Tl line framing, needs no extra bandwidth.
This solution does not work with other types of trunk interfaces and for packets that are not 24 bytes long. Fractional Tl and framed El (El is the 2.048 Mbps trunk defined by CCITT G.703 standard) can supply byte alignment but do not have 24 bytes per frame for the packet alignment needed by the IPX packet protocol. Unframed El (Britin's Mega-Stream), V.11 (Britain's Kilo-Stream and France's Transfix) and V.35 sub-rate trunks do not have either byte or packet alignment signals in their physical interface.